Tunable circuit



March 1, 1949. HE. OVERACKER V TUNABLE CIRCUIT Filed Aug. 8, 1945 3 IOll I O O I l O FIG.3

2 Sheets-Sheet l FlG.l

INVENTOR HORACE E. OVERACKER ATTORNEY March 1, 19499 H. E. OVERACKER2,453,417

TUNABLE CIRCUIT Filed Aug. 8, 1945 2 Sheets-Sheet 2 H 23 20 INVENTORHORACE E. OVERACKER BY ATTORNEY Patented Mar. 1, 1949 TUNABLE CIRCUITHorace E. Overacker, Cambridge, Mass, assignor to the United States ofAmerica as represented by the Secretary of War Application August 8,1945, Serial No. 609,648

5 Claims. (Cl. 178-44) This invention relates generally to tunable radiofrequency circuits. More specifically it relates to a tunable circuitformed by a section of transmission. line tunable over a wide band ofhigh frequencies which is useful in oscillator, amplifier, and otherapplications of tuned circuits and to the means for tuning the circuit.

Use of parallel and concentric transmission lines as resonant circuitstuned by the adjustment of shorting elements between the conductors ofthe transmission lines is well known to prior art. Since the frequencyto which such a circuit is tuned is substantially inversely proportionalto the length of the line, the lower the frequency it is desired toreach the longer the line must be. However, such frequencies, althoughrelative low, are too high for the use of ordinary elements havinglump-ed inductance and capacity. Thus at relatively low frequencies aline of extreme length would. be required. Even if the low frequency endof the operating range extended into the frequencies where lumpedreactance elements could be used, the entire range could not be coveredby a single piece of apparatus because the higher frequencies wouldrequire the use of transmission lines.

An added disadvantage in the use of transmission lines of the prior artarises because movement of the tuning element is rectilinear and can begauged with the rotational movement of other controls only bycomplicated mechanical linkage which introduces back lash and otherdifiicult'es. It is an object of the present invention to devise acircuit comprising a transmission line of short physical length withtuning means that make it tunable over a wider range of frequencies thanhas been accomplished by comparable transmission lines of the prior artand that will operate satisfactorily over the entire range.

It is a further object of the invention to increase the electricallength of a transmission line of given physical length and to arrangethe transmission line in a compact form.

It is a further object of the invention to devise positively controlledtuning means which over the entire tuning range, make positive anddirect electrical contact between the movable parts.

These objects are accomplished by disposing the transmission line in acircular arrange ment, shorting the line by a rotating contact,increasing its electrical length by use of high dielectric constantbetween its conductors for a portion of the length of the line, and by avaritrio constant 2!.

able capacitor, controlled by the same means as those controlling therotating contact.

Other objects, features and advantages of this invention will suggestthemselves to those skilled in the art and will become apparent from thefollowing description of the invention taken in connection with theaccompanying drawings in which:

1 is a plan view partially in cross-section of apparatus embodying theinvention;

Fig. 2 is. a combined cross-section along line 2-2 of Fig. l with afragmentary isometric view.

3 is a view showing the details of the control arm, its.contacts, andthe condenser cam; and

4-. is an isometric view of the open end of the transmission line alsoshowing the variable condenser structure and terminals.

Referring to Figs. 1 and 2, the lower half of the outer conductor of thetransmission line formed by an annular channel of semicircularcross-section formed near the circumference of the metallic element Hwhich forms the framework fo the support of the entire structure. Theupper half of outer conductor i0 is formed in an annular ring i3 fittingon framework H. The circular channel thus formed does not extend for acomplete circle but starts at back plate i l and extends for all but afew degrees of a complete circle to the open end terminating at terminaliii. The opening between the permanently shorted end it and terminal i5is large enough to permit the necessary connections to outside circuitsor, ,when desired, to mount a vacuum tube or similar apparatus. Theinner conductor it which is of sufficient strength and cross-section tosupport its own weight over a considerable space is supported at one endby a back plate in and screw ii and at the other end by attachment tothe condenser structure 19 which in turn is rigidly attached toframework H. Conductor it may be directly connected to framework 6 i byany suitable insulating bracket if no condenser is used. The innerconductor may be solid or hollow as desired. The inner conductor hasinserted along the inner circumference of the circle forms a contacttrack 20 oi metal, such as M nel metal, which combines good contactp'operties with favorable wearing properties.

For a portion of the length of the transmission line, the innerconductor i6 is surrounded and the space between it and outer conductorH1 is completely filled with a material of high dielec- This dielectricmaterial may be in two solid pieces, an upper half and lower half, or insegments.

Outer conductor I is provided with a longitudinal slot 22, comparable inwidth to the diameter of inner conductor I8, for its entire length alongits inner circumference. There is a contact track 23, 23 inserted ineach face forming this slot. This slot extends through dielectricmaterial 2| and permits contact with contact track and contact tracks23, 23 by contactors carried by arm 24 in a manner to be described indetail hereinbelow.

The effective electrical length of the transmission line and thereforethe resonant frequency of the circuit is varied by changing the positionof the contactors 25, and 26, 26 along the line and short circuitinginner and outer conductors. This is accomplished by means of control arm24 Fig. 3, bearing spring contactors 2E, and 25, 26. These contactorsare made of a metal, such as a beryllium copper alloy, which combinesgood spring action, good contact conductivity, and favorable Wearingcharacteristics against the material used for the contact tracks.Contactor for the entire length of contact track 28, makes contacttherewith as do contactors 25, 2a with respect to tracks 23, 23.

Control arm 24 is mounted and rotates with rotatable shaft 21 at thecenter of the circle formed by the transmission line. There is mountedon this shaft condenser cam 23. For convenience of manufacture, controlarm 24 and condenser cam 28 may, if desired, be made integral with abushing 29 by which they may be mounted on shaft 21. Shaft 21 issupported by bearing 30.

Condenser structure 19 is mounted on framework II by a bracket 31 nearthe open end of the transmission line. The top piece it stops before itreaches the place when condenser i9 is mounted. Condenser i9 is avariable condenser consisting of fixed plates 32 and movable plates 33.It is connected across the open end of the transmission line with fixedplates connected to inner conductor i6 and movable plates to outerconductor II]. The area of interleaved plates is varied by rectilinearmotion between fixed plates and movable plates. This motion is impartedto the movable plates in one direction by the face 34 of cam 28 actingon push rod 35 provided with ball bearing 36 and in the other directionby spring 31. The movable plates are mounted against and guided bysupporting block 38. Block 38 is made of insulating material and alsoserves to support fixed plates 32 on the condenser framework 3!. Block38 also provides insulation between inner conductor l6 which as beforestated is connected by attachment R8 to the condenser structure andouter conductor ill. The condenser is provided with a laterally bearingspring 39 to hold the movable plates in proper lateral position and ingood electrical contact with outer conductor if). The condenser and camare designed so that at the high frequency end of the operating rangethe movable plates are retracted a suflicient distance from the fixedplates to preclude the possibility of appreciable end effect and so thatfull capacity is in the circuit when control arm 24 is at the limit ofits travel at the permanently shorted end.

From the foregoing description of the physical arrangement of theapparatus it is obvious that, for a given physical length oftransmission line, the circular arrangement provides a much more compactdevice than would be possible if the transmission line were laid out inthe conventional manner, that is, a straight line. For a given physicallength, the electrical length is further increased by the use of thedielectric 2i and the capacitor l9 as will now be explained.

Dielectric material 2| is placed in a portion of the line at the shortedend 14 of the transmission line and cam 28 is so arranged on shaft 21that capacity from capacitor i9 is added only after the rotatingcontactors 25 and 26, 26 are contacting the portion of the transmissionline including dielectric 2|. As is Well known, the presence of materialhaving a dielectric constant between the conductors of a concentric lineincreases distributed capacity and electric energy travels through thedielectric material at a slower rate than it would travel through air.These phenomena have the effect of increasing the electrical length ofthe line.

As control arm 24 is rotated from the open end at terminal IS, theelectrical length of the transmission line is increased and its resonantfrequency decreased at a rate dependent solely on the physical length ofthe line between its open end and the shorting contactors 25 and 28, 26which is determined by the angular position of control arm 22 As soon ascontrol arm 24 contacts the portion of the transmission line where innerconductor I6 is surrounded by dielectric material 2! the rate at whichthe electrical length of the transmission line increases andconsequently the rate of decrease of its resonant frequency due to theincrease in distributed capacity is accelerated.

Cam 28 is mounted on shaft 21 or bushing 29 in such angular relation tocontrol arm 24 that no capacity from condenser i9 is introduced into thecircuit of the transmission line until control arm 23 has travelled oversome portion of the transmission line. In practice, it has been usual todelay the introduction of capacity from condenser 19 until after contactarm 2% has traversed an appreciable length of line containing dielectricmaterial 2!. Introduction of capacity from the condenser into thecircuit has a similar but separate effect to that of the dielectricmaterial on the rate of increase in electrical length of thetransmission line and decrease in its resonant frequency. Since theseeffects are concurrent and additive that rate is even more greatlyaccelerated.

It is thus seen that the operation of the line at the high frequency endof the operating range is unaffected by the capacity added by thedielectric material 2| and condenser 19 but that these elements have, atthe low frequency end, the eiiect of greatly adding to the electricallength of the line and thus extending the low frequencies over which itcan be tuned and extending the operating range.

Due to the compact form and convenient arrangement of the parts, thedevice is readily connected to other circuits. Thus, if it is desired touse it as a tunable circuit in a vacuum tube oscillator, the tubeelectrodes can be connected to the outer conductor at terminal [5 and tothe inner conductor at attachment I8, or any suitable place on fixedplates 32. If inner conductor i6 is hollow it may provide a convenientconduit through which direct current supply leads may be lead. Thedevice may also be combined with other devices of the same type andganged with the other devices to be tunable over the same frequencyrange or over a different but related frequency range as is desired forproducing an intermediate frequency.

While there has been here described what is at present considered to bethe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention.

What is claimed is:

1. A tunable radio frequency circuit comprising a concentrictransmission line arranged in a circular form and having an innerconductor and an outer conductor, said line being open at one end,material having a high dielectric constant disposed between said innerconductor and said outer conductor along a portion of said line remotefrom said open end, a slot disposed along the length of the innercircumference of said outer conductor and the inner portion of saiddielectric material, contact tracks on the portion of said outerconductor adjacent to said slot, a contact track along the innercircumference of said inner conductor, contactors mounted on a rotatablearm disposed to contact said contact tracks whereby said line may beshorted by said conta-ctors anywhere along its length, a variablecapacitor connected across said inner and outer conductors operated by acam mechanically connected to said rotatable arm, said cam disposed sothat no capacity from said capacitor is introduced into said circuituntil said contacts have travelled over a portion of said line includingsaid dielectric material.

2. A tunable radio frequency circuit designed to operate over a widerange of frequencies comprising a circularly arranged concentrictransmission line having an inner conductor and an outer conductor,movable means for short circuiting said conductors, means for extendingthe electrical length of said line comprising a material of highdielectric constant disposed between said inner conductor and said outerconductor along a portion of said line used when said circuit is tunednear the low frequency end of said operating range, additional variablecapacitor means controlled by the movement of said movable shortcircuiting means for introducing additional capacity into said circuitwhen it is tuned near the low frequency end of said operating range.

A tunable radio frequency circuit comprising a circularly arrangedconcentric transmission line circumferentially interrupted to provideterminal ends therefor, variable capacitor means connected across oneterminal end thereof, distributed capacitor means disposed along aportion of line adjacent the other terminal end thereof, shortcircuiting means adjustable along the length of said transmission line,means operative by said short circuiting means to vary the capacity ofsaid variable capacitor means and to render said variable capacitorinoperative when said short circuiting means is adjusted along theportion of said line that does not include said distributed capacitormeans.

i. The circuit according to claim 3 wherein adjustment of saidshort-circuiting means along the portion of said transmission lineincluding said distributed capacitor means renders operative saidvariable capacitor.

5. The circuit according to claim 4 wherein the capacity introduced intosaid circuit by said distributed capacitor and said variable capacitoris additive.

HORACE E. OVERACKER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,121,855 Buschbeck June 28, 19382,292,254 VanBuren Aug. 4, 1942 2,379,047 Thomas June 26, 1945 2,400,597Peterson May 21, 1946

